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Impact of Forcing/Coupling on Atmospheric and Oceanic Forecasts

Herve Giordani

Meteo-France/CNRM/GAME42 Av. G. Coriolis 31057 Toulouse Cedex 01, France

[email protected]

1 Abstract

The south-eastern France is prone to heavy rain events during the fall season. For these extreme precipitat-ing events, the Mediterranean Sea fuels the atmospheric boundary layer in heat and moisture and contributessometimes to accentuate flooding. This study aims at examining on what extent the SST, the surface heat fluxesand the coupling of an oceanic mixed-layer to an atmosphericmodel affect severe Mediterranean mesoscaleconvective systems. The feedbacks on the ocean mixed-layerare also presented. This study could help us atdefining the best coupling strategy for mesoscale atmospheric forecasting.

2 Introduction

It is well known that the spatial distribution ofSeaSurfaceTemperature (SST) and associated surface heat / mo-mentum fluxes induces strong modifications of the flow in the Marine Atmospheric Boundary Layer (MABL)over a large range of spatial scales, specially at mesoscale. For example, intense thermal gradients frequentlyform within the MABL in presence of major oceanic currents, such as the Gulf Stream, Kuroshio and Azorescurrents, in response to the large differential surface energy fluxes. The MABL takes therefore a baroclinicstructure which induces ageostrophic circulations dynamically similar to a sea breeze (Khalsa and Greenhut,1989). Such SST-induced MABL baroclinicity can trigger explosive cyclogeneses at middle latitudes (Giordaniand Caniaux, 2001) or secondary circulations in the MABL as strong as the synoptic flow either in equato-rial/tropical regions (Zhang and Busalacchi, 2008) or at middle latitudes (Giordani and Planton, 1998). Intensesurface fluxes associated with high SST can also destabilizethe MABL, increase the convection activity andthe cyclogenesis intensity during its initial stage (Homaret al., 2003).

However, it has also been shown that not only the SST has to be considered when analysing the influence of theocean on meteorological phenomena, but also the upper oceanthermal structure. As instance, it has been foundthat the Atlantic hurricanes intensify when passing over deep warm ocean cores (Goni and Triganes, 2003).Some studies have also shown that tropical cyclone are more sensitive to the mixed-layer heat content than tothe SST (Wada et al., 2007). As consequence, coupledOcean-Atmosphere (OA) models are more suited tosimulate such events with more realism because the oceanic heat content evolution is taken into account. Fewstudies have examined the interaction between the Mediterranean sea and the mesoscale atmospheric eventsusing a kilometric-scale OA coupled system. They have mainly focused on the study of Bora events overthe Adriatic sea. Coupled high-resolution simulations performed on these situations have shown that the fullinteractive coupling provided more accurate SST than the forced ocean simulations (Loglisci et al., 2004; Pullenet al., 2006).

Heavy precipitating events frequently occurred over the Western Mediterranean basin during the fall season,often leading to floods and damages. Heavy rainfall events are characterized byMesoscaleConvectiveSystems(MCS) generating large precipitation totals that could persist during several days. The synoptic patterns lead-ing to these events are generally well known, but the intensity and the stationarity of these systems are mainly

ECMWF Workshop on Ocean-atmosphere interactions, 10-12 November 2008 113

GIORDANI , H.: IMPACT OF FORCING/COUPLING ON ATMOSPHERIC ANDOCEANIC FORECASTS

controlled by various mesoscale factors (Mediterranean Sea surrounding mountains, cooling beneath the con-vection due to precipitation evaporation, low-level wind convergence and moisture transport; Ducrocqet al.,2002, 2003; Nuissieret al., 2008) including the Mediterranean sea. Indeed the Mediterranean Sea contributesto fuel these heavy precipitating systems in heat and moisture (Millan et al., 1995; Romeroet al., 1997; Pastoret al., 2001; Homaret al. 2003; Lebeaupinet al., 2006) and could also play a part in amplifying floods whenthe swell and waves perturb the river runoff.

This paper presents a sensitivity study of three Mediterranean MCSs during the fall season to thei) SSTii) thesurface fluxes parameterization andiii) the coupling of an ocean mixed-layer model to an atmosphericmodel.The mixed-layer sensitivity to the surface fluxes is also presented. Both oceanic and atmospheric responses areinvestigated in order to identify the key oceanic parameters for MCS forecasting.

3 Experimental Design

3.1 Heavy Precipitation events

The three selected heavy precipitation events are the same than in Lebeaupin et al. (2006). The events, namedfrom the French department1 that received the most important precipitation amount, occurred on:

• 12-13 November 1999 (hereafterAudecase),

• 8-9 September 2002 (hereafterGard case),

• 3 December 2003 (hereafterHerault case).

These three cases are major torrential rain events that occurred over South-eastern France; huge precipitationtotals (more than 500 mm in less than 24 hours), mainly induced by quasi-stationary mesoscale convectivesystems, have been recorded. These events resulted in majorflash floods with more than 30 and 20 fatalities,respectively. A comprehensive description of these cases can be found in (4), (5) and (3), only a brief overviewis given here.

The Aude case was characterized by an upper-level low-pressure area centred over Spain on 12 November 1999at 1200 UT that induced a vast southerly flow from North Africato southern France. Within the warm air masstransported by the southerly flow, surface lows formed and accelerated the low-level easterly to south-easterlyjet over the Mediterranean Sea with winds of more than 25 m s−1. The convergence is also enhanced by thedeflection of the low-level flow by the Southern Alps. The surface rainfall totals reached the maximum valueof 624 mm in less than 48 hours in Lezignan-Corbieres (Audedepartment).

The Gard case was characterized by an upper-level cold pressure low centred over Ireland and extending tothe Iberian Peninsula during the morning of 8 September 2002, resulting in a southwesterly diffluent flowover South-eastern France. Associated with the upper-level low-pressure area, a surface cold front undulatedover western France. An intense southerly low-level flow, conditionally unstable, established over the FrenchSouth-eastern coast. Convection organized in a MCS and became quasi-stationary and affected mainly the Gardregion. The precipitating system has stayed over the regionfor almost 24 hours. During this period, surfacerainfall totals reached until 691 mm in 24 hours recorded near the Ales town (Gard Department).

The Herault case was characterized by an upper-level low-pressure area centred over Spain that established asoutherly flow over southern France, and, a slow moving surface frontal system with embedded convection.The cold surface front stationned over the Gulf of Lions areaand South-eastern France from 1 to 4 December2003. The 3 December 2003, which was the most convective day,daily rainfall totals reached about 150 mm.Low-level winds also intensified during this day: easterly wind gusts up to 25-40 m s−1 over the Gulf of Lions

1A department is a subdivision of France administered by a prefect.

114 ECMWF Workshop on Ocean-atmosphere interactions, 10-12 November 2008


were observed and a strong swell, with associated beachcombers waves reaching 9 meters, disturbed the riverwater run-off to the sea. The flooding resulted in a major floodof the Rhone river.

4 The Numerical Tools

4.1 The MESO-NH Atmospheric Model

Atmospheric simulations were performed with the non-hydrostatic mesoscale MESO-NH model (13). Thesame model configuration as in Lebeaupin et al. (2006) is usedwith two interactive nested grids running athorizontal resolution of 9.5 km and 2.4 km respectively (Figure 1), centered over the mesoscale system withlocation and size depending on the studied case. The sub-grid scale convection is parameterized following(2) at 9.5 km resolution whereas no convective scheme is used at2.4 km. We focus in the following on the2.4km-domain that covers approximately a 600km x 600km areaaround the Gulf of Lions. The atmosphericinitial states are provided by the ARPEGE analyses.

4.2 The 1D Ocean Model

The one-dimensional kinetic energy model described by Gaspar et al. (1990) has been used for simulationsof the oceanic vertical mixing. By analogy with the atmospheric turbulence, this model includes a prognosticequation for the turbulent kinetic energy with a 1.5 closure. The prognostic variables are the temperature, thesalinity and the current defined on 40 vertical levels, spaced by 5 m near the air-sea surface up to 1000 m for thedeeper ocean. The ocean temperature and salinity are initialized by a MERCATOR analysis (1). The currentare initially supposed to be null.

4.3 The Ocean-Atmosphere Coupled System

MESO-NH can be coupled to the ocean mixed-layer model at eachgrid point of the atmospheric sea domainthrough the surface module SURFEX. Given that the ocean model is attached to the atmospheric horizontalgrid, no coupler is required. When the coupling is activated, the atmospheric model supplies the ocean modelwith forecasts of radiation, turbulent heat and momentum fluxes and precipitation rates. The ocean modelupadates the SST that is used as the bottom boundary conditions of the atmospheric model at the couplingfrequency.

Forced and coupled simulations of the Aude, Gard and Herault cases were carried out with this system over thedomain shown Figure1.

5 Forced Simulations

5.1 MCS Sensitivity to Sea Surface Turbulent Fluxes

Figure 2 displays the sensible (H) and latent (LE) heat fluxes for ORI,ECUME and COARE experimentsduring the Aude case. ORI (Louis, 1979), ECUME (Weill et al.,2003) and COARE (Fairall et al., 2003) arethree surface fluxes parameterizations used in the atmospheric simulations. The differences are especially largeunder the low-level jet. Indeed, the Aude case is characterized by strong southeasterly/easterly low-level jets(≥25 m s−1) converging over the Gulf of Lions. Associated with the strong convergent winds, large latent heatfluxes over the sea are therefore simulated, until more than 500 W m−2 near the French coasts at 0600 UT, 13November 1999 in ORI experiment. The COARE and ECUME experiment simulates latent heat fluxes in that



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Figure 1: Simulation domains. Large domain at the resolution 9.5 km (left). Small domain in the WesternMediterranean basin at the resolution 2.4 km (right).

area that can be 200 W m−2 weaker than the ORI ones. In average over the sea domain the difference reaches60W/m2. The sensible heat fluxes are not significantly different between the two parameterizations.

For the three cases, the COARE3.0 and ECUME parameterizations result in a reduction of the maximum ofprecipitation by 5 to 10 %. Weaker latent heat fluxes and less unstable low-levels result in a decrease by about10% of the precipitation amounts and of its maximum without having a significant impact on the location ofthe convective system in the Aude case (Fig.3). Although the mean precipitation surface totals on the domainare very close between the two experiments, the local maximum value of 24h-precipitation totals is lower by30 mm in COARE and ECUME compared to ORI (Fig.3). Scores against the observations show that for thatcase the 24h-precipitation totals simulated by COARE are slightly improved, with reduced bias and root meansquare and higher correlation (not shown).

5.2 Oceanic Mixed-Layer response to MCS forcing

The strong heat and momentum surface fluxes associated with MCSs result in strong cooling and deepeningof the mixed-layer typically of 0.5oC/dayand 30m/day, respectively (Lebeaupin et al., 2006). In order orderto analyze the precipitation effect on the mixed-layer, twosimulations were performed by desactivating andactivating the precipitation on the Herault case (Fig.5). Without precipitation (Figure4), the mixed-layerstrongly deepens up to 200m in accordance with the strong surface heat loss and surface wind-stress. Whenactivating the precipitation, the rain band at sea rapidly decreased the mixed-layer depth by decreasing thesalinity in the oceanic upper layers. A marked internal and stably boundary layer developed as shown on theFigure6. The underlying question is what impacts could have such oceanic structures on surface fluxes andfinally on MCSs ?

5.3 Effect of Time Frequency on the Oceanic Mixed-Layer

From the atmospheric simulation, three sets of atmosphericforcing for the ocean model were defined. Thefirst one has a time resolution equal to the time step of the ocean model. The instantaneous fields of the atmo-spheric simulation are thus provided to the ocean model every 5 minutes (OWO05m) then, we averaged theatmospheric fields over 1 hour (OWO01h) and 6 hours (OWO06h). The averaged values are used to drive theocean model during each 1 hour and 6 hours period, respectively. Thus, the energy amount exchanged betweenthe ocean and the atmosphere is the same at the end of the simulation for the three sets. Large differencesin surface density between the experiments OWO06h and OWO05m are found beneath the heaviest rainfalland under the low-level jet (Fig.7). In OWO 06h experiment, the low-level jet intensification after 14 UT(until locally 30 m s−1) is not really captured in the 6-hour averaged wind stress. The currents simulated by



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Figure 2: Aude case: Simulated surface wind superimposed tothe sensible heat flux (left) and to the latentheat flux (right). Surface fluxes simulated by ORI (a); ECUME (b) and COARE (c).

Figure 3: Aude case : 24h-accumulated precipitation differences between the simulations using the surfaceparameterizations ECUME and ORI (ECUME-ORI, left) and COARE and ORI (COARE-ORI, right).



Figure 4: Herault case : Simulated oceanic mixed-layer depth without precipitation.

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Figure 5: Herault case : 24h-accumulated precipitation field.

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Figure 6: Herault case : Simulated oceanic mixed-layer depth with precipitation. The shallow mixed-layerand the rain band at sea in the Gulf of Lions shown in Figure5 are at the same location.



CURRENT(OWO_06h) 0.25 m/s

DENSITY (OWO_06h − OWO_05m)

Figure 7: Herault case : Surface water density difference between the simulations using the surface forcingfrequency 6h and 5 min. The surface wind-induced current is superimposed to the difference density field.

OWO 06h are consequently weaker than those simulated by OWO01h and OWO05m. The intense rainfallrates are also smoothed and replaced by moderate rainfall rates in OWO06h, so the fresh-water input by theprecipitation is more easily integrated to the ocean mixed layer by the turbulent vertical mixing in OWO06h.On the opposite, in OWO05m or OWO01h, heavy precipitation that reach 60 mm/h locally, perturb or evenstop during several hours the ocean vertical turbulent mixing, and forms relatively thin internal boundary layernear the interface with relative low-salty water.

6 Impact of Coupled Simulations to Heavy Rainfall Events

As shown by Lebeaupinet al. (2006), the intensity of the atmospheric deep convection and the precipitationtotals are sensitive to the total energy available in the atmospheric low-layer over the sea. As SST decreaseswith time during the full interactive mode (CPLCO), it induces a steady decrease of the air-sea heat fluxesand therefore of the energy available from the Mediterranean Sea for the atmospheric deep convection. Con-sequently, the precipitation systems are slightly less intense and generate weaker surface rainfall totals. Forexample, for the Gard case (Fig.6), the maximum 24-h precipitation amount in CPLCO is 24mm weaker thanthe one simulated by the forced mode CFCOA. The accumulated precipitation patterns are however very similarbetween the two experiments.

To sum-up, the comparison between the one-way forcing and the two-way coupling shows that the full two-waycoupling tends to attenuate the two boundary layer responses. During the Mediterranean heavy precipitationevents, large sea-air fluxes prevail and cools the ocean mixed layer which in turn tends to lower sea-air fluxesin the two-way coupling. For the short-range simulations performed here, the differences between the fulltwo-way coupled and the one-way forced modes are however relatively small.

7 Conclusions

This study has shown a significant sensitivity of mesoscale convective system to the surface fluxes and tothe SST because both these elements drive the atmospheric stability (CAPE) at least in the boundary layer.Using a higher-resolution SST field produces smaller-scalepatterns in the surface heat fluxes fields, but hasminor impacts on the convection and on low-level jets (not shown, see oral presentation or Lebeaupin et al.,2006). Also a modification in SST field can induce different atmospheric dynamical responses according to themeteorological case (not shown, see oral presentation or Lebeaupin et al., 2006).

The oceanic mixed-layer has also shown a great response to strong surface heat loss and surface wind stress as-



CFCOA CPLCOmm

Figure 8: Gard case : Simulated 24h-accumulated precipitation field in the forced mode (left) and in thecoupled mode (right).

sociated with mesoscale convective systems. Particularly, the development of fresh and stably internal boundarylayers induced by intense precipitation rates is strongly affected by the surface forcing frequency.

The full two-way coupling tends to attenuate the oceanic andatmospheric boundary layer responses comparedto the one-way mode. As a result, convective activity and rainfall intensity are weaker in the coupled simulationscompared to the forced simulations. The limited feedbacks on the heavy precipitating systems must be temperedby the short duration of the simulations (24 h).

8 Major Issues for Coupled Mesoscale Models

Although this study has shown a limited impact of the ocean-atmosphere coupling on three mesoscale convec-tive cases, this result cannot be considered as a general conclusion. A lot of other various atmospheric situationshave to be studied to conclude on the relevance of the coupling at mesoscale. Nevertheless the results of thisstudy help us to identify some key points of the coupling at mesoscale which are still open questions : whatresolution for the SST fields ? what surface fluxes parameterization ? what coupling frequency ? what verticalmixing (diffusion/convection) in ocean models ? what wouldbe the added value for operational atmosphericforecasting to couple an oceanic mixed-layer model ?

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